Cable operators are interested in deploying high-speed data communications systems on cable television systems. In an effort to enable the definition, design, development and deployment of data-over-cable systems on an uniform, consistent, open, non-proprietary, multi-vendor interoperable basis, the cable operators have prepared a series of interface specifications known as Data-Over-Cable Service Interface Specifications (DOCSIS). The intended service will allow transparent bi-directional transfer of Internet Protocol (IP) traffic, between the cable system of the cable operator and customer locations, over an all-coaxial or hybrid-fiber/coax (HFC) cable network.
FIG. 1 illustrates a communications system 10 envisioned by DOCSIS. The communications system 10 includes a cable modem termination system (CMTS) 12 that interfaces with a wide-area network 14, such as the Internet. The CMTS 12 is coupled by a cable network 16 to a cable modem (CM) 18, which is coupled to a customer premises equipment (CPE) 20, such as a computer, by a CM-CPE interface 22. The CMTS12 and cable modem 18 provide a cable-based communication interface between the CPE 20 and the wide-area network 14, thereby allowing a user of the CPE 20 to send data to and receive data from the wide-area network 14. The advantages of such a cable-based communication system 10 is that the cable network 16 is already in place in most locations in the United States for television systems and the cable network 16 is capable of much faster data transmission rates than current systems employing public telephone lines.
Although it specifies many basic requirements of the communication system 10, DOCSIS does not specify the details of how those requirement are implemented. For example, DOCSIS specifies (page 14) the following rules (among others) that the cable modem 18 must follow when exchanging data between the cable network 16 and the CPE 20, but does not specify the hardware and/or software to be used by the cable modem 18 to satisfy the rules.
3.1.2.3.2 Forwarding
Cable modem (CM) forwarding in both directions MUST conform to the following general 802.1d guidelines:                Link-layer frames between a given pair of end-stations MUST be delivered in order.        Link-layer frames MUST NOT be duplicated.        Stale frames (those that cannot be delivered in a timely fashion) MUST be discarded.        